CODEWITHSUNDEEP
pythonpython-3.xlistset
Ismael Ouarit
Ismael Ouarit

Reputation: 1

Why is a list of strings shown as an example for a "hashable object"?

I am currently learning Python on codecademy and I came across this statement that says that lists are hashable (thus, they can be used as input for the update method when wanting to add multiple elements to a list):

The .update() method can add multiple elements.

# Create a set to hold the song tags
song_tags = {'country', 'folk', 'acoustic'}

# Add more tags using a hashable object (such as a list of elements)
other_tags = ['live', 'blues', 'acoustic']
song_tags.update(other_tags)

print(song_tags)

But searching what a hashable object was, I found out that lists are not hashable, so now I am confused. Is this a mistake or are lists hashable in this particular situation?

Upvotes: 0

Views: 39

Answers (1)

ForceBru
ForceBru

Reputation: 44848

Lists aren't hashable:

>>> hash([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> {["hello", "world"]}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

So yes, Codeacademy is incorrect here. set.update accepts any iterable as an argument:

>>> d = set(); d.update("hello"); d # string
{'h', 'l', 'o', 'e'}
>>> d = set(); d.update(['h', 'i']); d # list
{'h', 'i'}
>>> d = set(); d.update((h for h in "hello")); d # generator
{'h', 'l', 'o', 'e'}

The only thing that matters is that its elements be hashable, not the iterable itself.

Upvotes: 2

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